I operate a MultiPlus on a 314Ah, 48V, LiFePO4 battery. I feed the A/C, a Truma Aventa compact plus 2.gen of my camper van. It draws roughly 1.3kW at full load and is a modern, inverter driven device. The A/C controller shows an error message “unstable/distorted input frequency” on start-up and shuts down the A/C. The controller tries to restart several times and occasionally manages. It looks as though I operated at the limit of the supply voltage quality check of the A/C.
Are their parameter settings (AES is off) available in the MultiPlus set-up which improve the behaviour of the MultiPlus during start up or hardware devices (filters) which improve the wave form of the MultiPlus output voltage, taking into account that I run two inverters in a series configuration.
I would suspect that the inverter input stage is causing problems with the switched output of the inverter- something like the beat frequency between 2 independent switches operating near the same frequency. I would try either a larger inverter, or 2 operating in parallel. One other option might be to try a synchronous AC motor, with no load and a parallel capacitor across the AC output of the inverter, and see if that solves the problem. The motor will act as a spinning reserve for short current spikes.
Thank you very much, Mike. The problem shows when the load is minimal and the a/c is about to reach its set temperature. Using a filter is for sure a good approach. The idea with the no load synchronous motor is tempting as it might be lighter and require less volume which is very important for an RV. Did you ever design such a set up and can you suggest the size of the motor?
yes I did, using a 1.5HP motor from an old air compressor and a 10uF 400V cap. this provided sufficient energy to enable a 1500A/5.5V spot welder to work from a single 3k multiplus2. (10mS weld time). 2MP2-3K’s worked even better, so it was an instantaneous current surge. Solution was a little noisy, but worked.
Looking at a simplified circuit for the output stage of an inverter, it’s basically a class D amplifier, and the magnetic medium of the transformer, plus any low pass filtering, provide current when the drive transistors are not conducting (simplified). If the A/C causes problems at lower load, rather than full load, it may be that it is not maintaining a unity power factor and low harmonic load under those conditions.